@pico You can adjust the brightness even when it is off, so that it will remember the configured brightness the next time it is turned on. Execute the command I wrote under "setting brightness", adjusting node id and level as you wish.
The "options" is a bit map with currently only having one defined bit "ExecuteIfOff": "If this bit is set, commands in this cluster are executed and potentially change the CurrentLevel attribute when the OnOff attribute of the On/Off cluster is FALSE".
So, if you change OptionsOverride to 0 in the command, the command will be discarded in case the light is off. But if you leave it as 1, then the brightness will be modified in case the light is off.
You cannot write to the CurrentLevel attribute directly using a raw write since it is read only. You need to use one of the commands to adjust the level.